I asked about ATX power supply beacuse pretty much all decent ones are 450W+ so the effect with the LED is easy to spot. But you can try that with AT PSU as well, though the results might not be conclusive. The reset switch works as it should on your mobo and so the problem doesn't seem to be reset related.
BTW the JP2 and cable for the test card are to connect the buzzer on it in place of PC speaker. You can verify that by testing connection between those two on the card. No idea what the button is for, I've never used it.
Well it could be the BIOS got corrupted. Not by UV light, unless you kept the board out in the direct sun without the EPROM windows covered. Fluorescent light doesn't really erase these unless it's years of exposure. Or lamps used to kill germs. EPROMs sometimes just flip back to "1" - it's a natural process that takes years even for weak cells but it happens. The mobo should output some error codes even without RAM, so long the CPU works and the chipset is not completly dead.
Do you have something to dump the EPROMs? Or maybe another board to try swapping both?
This mobo uses Motorola RTC, MC14618A - a very common chip. I'm actually surprised there were any Suntac 286 with Dallas chips. Must've been pretty late models.